Infrared (IR) technology is one kind of wireless technology that is commonly utilized in electronic devices, like laptop computers, hand phones and PDAs (personal digital assistant) due to its low cost of implementation and high data transfer rates in the order of Mb/s. IR technology allows users to link to each other through various IR devices over a short distance. Generally, known IR devices comprise an IR transceiver for transmitting, receiving and processing an infrared signal. The IR transceiver including an IR sensor for capturing IR light and converting the same into an electrical signal, and a preamplifier for amplifying the electrical signal generated by the IR sensor. The power of the incident infrared light varies in orders of magnitude, depending on the distance between the source of the infrared signal, which can be e.g. a transceiver of another IR device or a remote control unit, and the sensor of the IR transceiver. As the distance increases, the infrared signal received by the sensor of the IR receiver decreases in an exponential manner. Furthermore, IR devices usually operate in conditions where there is ambient light. Such ambient light will reduce the Signal-to-Noise Ratio (SNR), and the sensor of the IR transceiver must still be able to detect the desired infrared signal despite the overwhelming photocurrent generated by the sensor corresponding to ambient light.
When the IR sensor captures an infrared signal, it generates a photocurrent, which is proportional to the signal strength. The photocurrent generated by the IR sensor needs to be amplified, and a transimpedance amplifier is used as the preamplifier. The transimpedance amplifier used in said known preamplifier is desired to be a low noise amplifier and should provide for a high gain. However, when using IR devices in the abovementioned reasonable range of distances, the intensity of the incident infrared light captured by the IR sensor varies typically in a dynamic range of about 100 dB (3.6 μW/SR to 500 mW/SR). Therefore, the intensity of the photocurrent generated by the IR sensor in response to the incident IR light and input to the preamplifier has also a dynamic range in the order of 100 dB. In contradistinction, the dynamic range of a typical transimpedance amplifier is only between 40 dB to 60 dB, beyond which the amplifier loses its linear properties and enters into saturation. As such, the transimpedance amplifier is not able to accommodate the wide dynamic range of the photocurrent generated by the IR sensor without losing its linearity.
A typical implementation of the receiver part of a known IR transceiver is shown in FIG. 1. The IR receiver comprising an IR sensor 100 including a photodiode 130, and a preamplifier is shown in FIG. 1. The preamplifier is implemented by a transimpedance amplifier 200 having an operational amplifier 210, a feedback resistor Rf 220 and a feedback capacitor Cf 230.
There are many constraints in designing the transimpedance amplifier 200 of this IR receiver. The lower bound of the dynamic range of the transimpedance amplifier 200 is determined by the level of the input referred noise of the transimpedance amplifier 200. Any signal with a signal level below the lower bound of the dynamic range will be very difficult to detect, as the SNR is very low in this range. The upper bound of the dynamic range of the transimpedance amplifier 200 is determined at the input signal level when the transimpedance amplifier 200 becomes saturated and the gain is no longer linear. Hence the dynamic range in which the transimpedance amplifier 200 should be operated must be between the upper and the lower bound so that on the one hand a low level input signal can be detected, and on the other hand a high level input signal is still amplified with a linear gain.
Further, the bandwidth requirement places a large constraint on the design of the transimpedance amplifier 200. The bandwidth of the transimpedance amplifier is approximated byBW≅1/(2πCfRf),where Rf is the feedback resistor 220 connected between the inverting input and the output of the operational amplifier 210 and Cf is the feedback capacitor 230 connected in parallel with the feedback resistor 220.
The desired operational bandwidth should not be too narrow so as to avoid distortions such as intersymbol interferences, jitters, etc. On the other hand, the bandwidth should not be too wide, either, as a wide bandwidth would reduce the SNR. Therefore, the values of Cf 230 and Rf 220 should be carefully chosen to obtain an optimal operational bandwidth. However, this is not so easy as the values of Cf 230 and Rf 220 themselves are also subject to various constraints.
The photodiode 130 of the IR sensor 100 is desired to have a large surface area and thereby a large sensing area in order to achieve a high signal strength resulting in a good SNR. However, a large photodiode will inevitably introduce a high parasitic capacitance Cpd 110 (also known as the pin capacitance) as the parasitic capacitance 110 is directly proportional to the size of the photodiode 130. Depending on the bias condition of the transimpedance amplifier 200, the Cpd 110 can be as high as 10 pF to 15 pF. With such a high Cpd 110, the circuit might have stability problems. In addition, the feedback capacitor Cf 230 needs to be designed as large as possible to ensure stability of the transimpedance amplifier 200. However, increasing the value of the feedback capacitor Cf 230 also decreases the bandwidth, resulting in increased distortion of the input signal.
Furthermore, the gain of the transimpedance amplifier 200 is directly proportional to the value of the feedback resistor Rf 220. The value of Rf 220 should be high on the one hand to achieve a high gain, but should not be too high on the other hand to avoid saturating the transimpedance amplifier 200 and narrowing its bandwidth. Reducing the value of the feedback resistor Rf 220 will reduce gain and inevitably increase the bandwidth of the transimpedance amplifier 200, resulting in reduction of the SNR. This poses another constraint on the value of the feedback resistor Rf 220.
As can be seen from above explanations, requirements regarding the sensitivity, dynamic range, Signal-to-Noise Ratio (SNR), bandwidth and stability of the IR receiver, and especially of its transimpedance amplifier, all pose constraints in designing the amplifier 200, and, therefore, many compromises have to be made.
One known implementation of a transimpedance amplifier is shown in FIG. 2 and uses an AGC (automatic gain control) unit 300 to vary the value of the feedback resistor Rf 220′ so as to avoid saturation of the transimpedance amplifier 200 by scaling the input photocurrent 120 generated by the IR sensor 100 to a range within 40 dB to 60 dB. One problem of this implementation is that since the dynamic range of the input photocurrent is very large as explained above, a feedback resistor Rf 220′ with a plurality of different values is needed to achieve a sufficient number of gain steps. Another problem is that this known implementation operates with high current values and has, therefore, high power dissipation.
Another known implementation of a preamplifier of an IR receiver is a diode clamping transimpedance circuitry 400 as shown in FIG. 3. The transimpedance circuitry 400 comprises a clamping diode 410 and a resistor 420 connected parallel to the clamping diode 410. The bandwidth of this diode clamping transimpedance circuitry 400 is approximated asBW≅1/(2πCpdR)wherein Cpd is the parasitic capacitor 110 of the photo-sensor 100 and R 420 is the transimpedance when the input current is low. One drawback of this known diode clamping transimpedance circuitry 400 is that it is only suitable for low speed implementations. Furthermore, there is no control over the gain of the preamplifier 200.